1. Field of the Invention
The present invention relates to the operation of a light modulator of the planar electrode type, and, more particularly, to a polarity reversing drive for a light modulator of the planar electrode type.
2. Descriotion Of the Related Art
Electro-optic materials are those whose optical properties change in accordance with the strength of an electric field established within them. These materials make possible an electrically controlled "electro-optic modulator". As used herein, the term "modulator" includes a device which changes the intensity of light in response to an applied electric signal. An "electro-optic modulator" includes a member formed of electro-optic material which receives plane polarized (linear) light and which changes the state of polarization of such light in response to an established electric field. An analyzer receives light from the member and blocks that light whose plane of polarization has not changed (no established electric field) while transmitting light when its plane of polarization has been changed by an established electric field. By changing the electric field in the electro-optic member, light which passes through the analyzer is modulated.
One example of an electro-optic material used in modulators is lanthanum-doped lead zirconate titanate (PLZT). Although PLZT is a preferred electro-optic material, it will be recognized by those skilled in the art that other electro-optic materials can also be used to change the polarization of light.
It is known that the voltage-to-transmissivity transfer characteristic of an electro-optic modulator can drift or change with time under constant illumination. This drift is generally believed to be caused by a "space charge"comprised of photogenerated or injected electrons or holes in the electro-optic material of the light modulator. The induced charges, (i.e., the space charge) drift under the influence of voltage applied between the electrodes attached to the electro-optic material. Thus, the net electric field between the electrodes varies in accordance with the strength and polarity of the space charge, and thereby causes a change in the voltage-transmissivity transfer characteristic of the light modulator.
Apparatus is known in the art for reducing the adverse effects of induced space charge. For example, U.S. Pat. No. 4,631,551 (A. B. Vergona) issued Dec. 23, 1986, entitled "Color Imaging Apparatus Using Electro-Optic Modulators" and U.S. Pat. No. 4,667,256 (A. B. Vergona) issued May 19, 1987, entitled "Circuit For Electro-Optic Modulators", both of which are assigned to the assignee of this invention, disclose respective feedback networks for controlling a light modulator with an applied unidirectional voltage.
An alternative approach is to reverse the polarity of the applied electric field at regular intervals. By way of example, U.S. Pat. No. 4,369,457 (R. A. Sprague), issued Jan. 18, 1983, entitled "Reverse Polarity Differential Encoding For Fringe Field Responsive Electro-Optic Line Printers"discloses a line printer having polarity-reversing drive voltage circuitry for inhibiting charge carrier accumulation in a light modulator. Drive voltage is switched at the line scan rate of the printer, which can be expected to be on the order of several cycles per second (Hz) or more for a modern day high-speed printer.
Through experimentation we have confirmed that reversing the polarity of the applied electric field at a rate of several hertz or more eliminates the aforementioned voltage/transmissivity drift problem for a light modulator of the type having electrodes of the so-called bulk, in-depth or channel type, i.e., electro-optic material is sandwiched between corresponding electrode pairs. However, with regard to an optical modulator having planar electrodes (i.e., corresponding electrode pairs on a common surface of the electro-optic material) we have found that operating the modulator by reversing the polarity of the applied voltage at a comparable rate (i.e., several hertz or more) provided an unsatisfactory solution to the "drift problem".
In an article entitled "The Mechanism of Applied-Field-Induced Remanent Birefringence In Transparent Ferroelectric PLZT Ceramics" by A.E. Kapenieks et al., published in Ferroelectrics Letter, 1982, Vol. 44, pages 189-196, it is reported in the case of an optical modulator having planar electrodes that a localized intense electric field occurs near planar electrode edges with injected carriers being trapped in the electro-optic material near the cathode electrode. It is further reported that this phenomenon has a dual disadvantage in that both the speed and the contrast ratio achievable with the light modulator are reduced.
Kapenieks et al. offer as possible solutions to this problem (1) thoroughly cleaning the surface of the electro-optic material with ethanol and fabricating the electrodes so that their edges are sharp, or (2) employing bulk electrodes instead of planar electrodes. The former solution would require at least one additional processing step whereas the latter would restrict a designer to a light modulator of the type in which electro-optic material is sandwiched between corresponding electrode pairs.